The 2-n.butyl-4-spirocyclopentane-1-[[2'-(tetrazol-5-yl)-biphenyl-4-yl]methyl] -2-imidazolin-5-one, or 2-n.butyl-3-[[2'-(tetrazol-5-yl)biphenyl-4-yl]methyl]-1,3-diazaspiro[4.4]n on-1-en-4-one, is a powerful angiotensin II receptor antagonist prepared by reaction of 2-n.butyl-1-[(2'-cyanobiphenyl-4-yl)methyl]-4-spirocyclopentane-2-imidazol in-5-one, or 2-n.butyl-3-[(2'-cyanobiphenyl-4-yl)methyl]-1,3-diazaspiro-[4.4]non-1-en-4 -one, either with tributyltin azide and triphenylchloromethane in xylene at reflux, by elimination of the triphenylmethyl protecting group and by isolation from a solution in ethyl acetate, duly dried (EP-A-0 454 511), or directly with tributyltin azide, in xylene at reflux and isolation from a solution in dichloromethane, duly dried (C. A. Bernhart et al., J. Med. Chem., 1993, 36, 3371-3380). The compound thus prepared, to which the formula (A) ##STR1## has been attributed, is presented in the form of stable, non-hygroscopic needles which can be stored and formulated without any degradation. However, it has been observed that the 2-n.butyl-3-[[2'-(tetrazol-5-yl)biphenyl-4-yl]methyl]-1,3-diazaspiro[4.4]n on-1-en-4-one obtained according to the above described processes, must be formulated with much care because the powder tends to remain attached to the apparatus, for example to the sieves, to the punches or to the mixer walls, due to its high electrostaticity.